Titas,
I have done something similar using RTC plain Java API.
Suppose that you have a IChangeSet object cs, you can retrieve the IFileItem objects for the previous and current versions of each file modified in the change set:

for (Object o: cs.changes()) {       
    IChange change = (IChange) o;
    if (change.kind() == IChange.MODIFY) {
        IVersionableHandle afterHandle = change.afterState();
        IVersionableHandle beforeHandle = change.beforeState();
        //skip folders
        if( !beforeHandle.getItemType().getName().equals("Folder") && !afterHandle.getItemType().getName().equals("Folder")){
            IVersionable afterVersionable = wm.versionableManager().fetchCompleteState(afterHandle, new SysoutProgressMonitor());
            IFileItem fileAfter = (IFileItem) afterVersionable;                   
            //skip metadata
            if(!fileAfter.getName().substring(0,3).equals("org") && !fileAfter.getName().equals(".project")){
                IVersionable beforeVersionable = wm.versionableManager().fetchCompleteState(beforeHandle, new SysoutProgressMonitor());
                IFileItem fileBefore = (IFileItem) beforeVersionable;
//....
            }
        }
    }
}

Then, you write on disk the "before" and the "after" version of each file:

//write before and after versions on disk
// ITeamRepository repo;
IFileContentManager contentManager = FileSystemCore.getContentManager(repo);
PrintWriter beforeOnDisk = new PrintWriter(outputDir+"before", "UTF-8");
InputStream isBefore = contentManager.retrieveContentStream(fileBefore, fileBefore.getContent(), null);
BufferedReader brBefore = new BufferedReader(new InputStreamReader(isBefore));
String lineBefore;
while ((lineBefore = brBefore.readLine()) != null) {
    beforeOnDisk.println(lineBefore);
}     
beforeOnDisk.close();
File beforef = new File(outputDir+"before");                 
PrintWriter afterOnDisk = new PrintWriter(outputDir+"after", "UTF-8");
String lineAfter;
InputStream isAfter = contentManager.retrieveContentStream(fileAfter, fileAfter.getContent(), null);
BufferedReader brAfter = new BufferedReader(new InputStreamReader(isAfter));
while ((lineAfter = brAfter.readLine()) != null) {
    afterOnDisk.println(lineAfter);
}
afterOnDisk.close();
File afterM = new File(outputDir+"after");

Now, it is up to you to decide which tool you use to compare the "before" and "after" versions and get the number of added/deleted/modified lines for each file.